* Upgrades the nixpkgs pin to 21.11.
* Removes nix code, sources, and attributes related to haskell.nix and
the code under pkg/hs.
* Continuous integration no longer builds (or packages/releases)
urbit-king (haskell).
* To continue to developing any haskell code under pkg/hs it's left as
a user exercise uto install and configure GHC/Stack.
As a consequence running nix-shell should be considerably more
performant and requires downloading or building less dependencies.
d3c5aa74c1 introduced a Docker image intended for deploying Urbit ships
This commit modifies the build.yml GitHub Actions workflow to build the
docker image as part of the push/PR workflow, and adds a
`release-docker.yml` workflow, which runs on any tag push `urbit-v*` and
pushes the image to Docker Hub, also synchronizing the README.
Co-authored-by: Josh Lehman <josh@urbit.org>
